Thyridopteryx ephemeraeformis The larval stage of the bagworm devours the foliage of many species of plants in the cypress family. The larvae hatch in spring and begin feeding on the needles. The larva constructs a bag that covers its entire body. The worm partially emerges from its bag to feed. When the leaves on a branch are completely consumed, the bagworm moves to the next branch. By late summer, the full-grown larva spins silken bands to attach the bag permanently and pupate inside. The mature female is a wingless grublike insect that remains in the bag. The male is a clear-winged black moth that is sometimes seen around lights at night in the fall.
Apply an insecticide labeled for this pest, following label instructions. Handpicking and destroying bags from fall to spring will reduce the number of overwintering eggs.
Leaf browning and shedding is a natural process similar to the dropping of leaves of deciduous trees. It is usually more pronounced on arborvitae (Thuja) than on other plants in the cypress family. Sometimes it takes place every year; in other cases, it occurs every second or third year. When growing conditions have been favorable the previous season, leaf shedding occurs over several weeks and is less noticeable. If the plant has been exposed to unfavorable conditions during the growing season, such as drying or a spider mite or insect infestation, however, leaf drop develops within a few days. Leaf drop is also caused by new growth shading older interior growth.
No chemical controls are necessary. Water regularly and feed with a fertilizer formulated for evergreen trees and shrubs. Provide full sun. Check plants for insects and mites during the growing season.
Argyresthia species Several species of insects known as leaf miners in the eastern United States and tip moths on the West Coast infest arborvitae, cypress, and juniper. Damage is unsightly, but plants may lose more than half of their foliage and still survive. The larvae spend the winter inside the leaf tips. When the weather warms in late spring, adult moths emerge and lay eggs on the leaves. The eggs hatch, and the larvae tunnel into the leaf tips, devouring the green tissue. The tips above the point of entry yellow and die. The larvae feed until late fall or through the winter until early spring.
Apply an insecticide labeled for these pests, following label directions. Trim and destroy infested leaves in fall and spring.
Several species of scales infest plants in the cypress family. They lay their eggs on the needles or bark, and in spring to midsummer the young scales, called crawlers, settle on needles and twigs. The small 1/10 inch), soft-bodied young feed by sucking sap from the plant. The legs usually atrophy, and with some types, a shell develops over the body. The types of scales that do not develop shells are conspicuous. Scales covered with a shell are less noticeable. Their shell often blends in with the plant, and the eggs are inconspicuous beneath the covering. Some species of scales are unable to digest fully all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew. A sooty mold fungus may develop on the honeydew, causing the leaves to appear black and dirty.
Control with an insecticide labeled for scales, or an insecticidal soap. During the winter, treat with a horticultural oil.
Spider mites, especially spruce spider mites (Oligonychus ununguis), are extremely destructive pests of evergreen trees. In the cypress family, they attack mainly arborvitae (Thuja) and Chamaecyparis, causing damage by sucking sap from the undersides of needles. As a result of their feeding, the tree''s green leaf pigment disappears, producing the stippled appearance. This symptom can be confused with certain types of damage caused by air pollution. Mites are active throughout the growing season but usually become most active in May and September. By midsummer, they have built up to tremendous numbers. Young plants may die the first season. If mites are uncontrolled for several years, older trees die progressively from the lower branches upward.
Control with an insecticide labeled for these pests, following label directions. In late winter, apply a horticultural oil.
Arborvitae are damaged by cold, drying winter winds, especially if temperatures are below freezing. These trees are commonly planted as windbreaks and in exposed areas where growing conditions may be unfavorable. Moisture is lost from the leaves more rapidly than it can be replaced by the root system. Cells in the leaves dry out and die. This condition is most pronounced when water is unavailable because the soil is dry or frozen. Leaves, along with twigs and branches, also die during early fall or late spring freezes when the plant is growing. Young succulent growth cannot withstand freezing temperatures.
Arborvitae are damaged by cold, drying winter winds. Prune out dead twigs and branches. Provide shelter for plants growing in extremely cold areas. Avoid fertilizing late in the season. During a dry fall, water thoroughly to reduce winter injury.